Antenna diversity techniques utilize two or more antennas to improve the quality and reliability of signals received or transmitted over a wireless link. A majority of wireless environments are urban environments in which signals are reflected along multiple paths before finally being received. Each of these bounces can introduce phase shifts, time delays, attenuations, and even distortions that can destructively interfere with one another at the aperture of a receiving antenna. Antenna diversity is especially effective at mitigating these multipath situations.
Furthermore antenna diversity allows the capacity of the system to be increased by using different bands or spatial regions within which to send or receive signals—for example by allocating different spatial regions for different channels allows the reuse of the same frequency band. Thus, antenna diversities (frequency, polarization, radiation pattern and spatial) are being explored for current and future multiple antenna smart wireless communication systems, such as LTE (long term evaluation) and MIMO (multiple input and multiple output).
Cellular standards like the third generation partnership program (3GPP) long term evolution (LTE), ultra-mobile broadband (UMB), high speed downlink packet access (HSDPA) and IEEE 802.16e (WiMAX) support multiple-input multiple-output (MIMO) wireless communication technology. MIMO uses multiple antennas at the transmitter and receiver along with advanced digital signal processing to improve link quality and capacity. Existing base stations use antenna arrays to provide transmit and receive diversity
Recently, studies on microstrip antennas have focused on frequency reuse and polarization diversity of the two-orthogonal polarizations to double the capacity of communication systems and reduce the multi-path fading of received signals in land-based mobile communications.
Moreover, dual-frequency microstrip antenna arrays, often realized through a multilayer architecture, have gained considerable interest. However, there have been some inherent challenges in the design and architecture of dual-polarized dual-frequency band microstrip antenna arrays.
Conventionally, a dual-polarized microstrip antenna is realized by feeding a patch at the two orthogonal edges. This feeding approach requires two feeding-networks for two individual polarization components, respectively. But it is difficult to allocate enough space to accommodate two sets of feeding networks if a dual-polarized array is to be employed within a limited allowable space. Strong mode coupling and high cross polarization is likely to occur. This problem exacerbated if active and passive circuits are required to be integrated into the feed-networks.
Furthermore, if a dual-frequency operation for the above dual-feed dual-polarized array is realized by multilayered architecture, the size and complexity of the array will be further increased.
Designers of antennas for mobile communications face significant challenges, particularly since antennas must be capable of covering as many bands as possible while being small in size and still having a high performance.